Augustin Cavalier
6f3f29c7dd
Suppose the following scenario: 1. Thread A holds a mutex. 2. Thread B goes to acquire the mutex, winds up in kernel waiting. 3. Thread A unlocks; first unsets the LOCKED flag. As WAITING is set, it calls the kernel; but instead of processing this immediately, the thread is suspended for any reason (locks, reschedule, etc.) 4. Thread B hits a timeout, or a signal. It then unblocks in the kernel, which causes the WAITING flag to be unset. 5. Thread C goes to acquire the lock. It sets the LOCKED flag. It sees the WAITING flag is not set, so it returns at once, having successfully acquired the lock. 6. Thread A, suspended back in step 3, resumes. Now we encounter the problem. Under the previous code, the following would occur. 7. Thread A sees that no threads are waiting. It thus unsets the LOCKED flag, and returns from the kernel. Now we have a mutex theoretically held by thread C but which (illegally) has no LOCKED flag set! 8. Some other thread tries to acquire the lock, and succeeds, for LOCKED is not set. We now have one lock owned by two separate threads. That's very bad! The solution, in this commit, is to (1) switch from using "atomic_or" to lock mutexes, to using "atomic_test_and_set", and (2) mandate that _kern_unblock_mutex must be invoked with the mutex already unlocked. Trying to solve the problem with (2) but without (1) produces other complications and would overall be more complicated. For instance, all existing userland code expected that it would set LOCKED, but then check LOCKED|WAITING. If _kern_mutex_unlock does not unset LOCKED, then whichever thread sets LOCKED when it was previously unset is now the mutex's undisputed owner, and if it fails to notice this, would deadlock. That could have been solved with extra checks at all lock points, but then that would mean locks would not be acquired "fairly": it would be possible for any thread to race with an unlocking thread, and acquire the lock before the kernel had a chance to wake anyone up. Given how fast atomics can be, and how slow invoking the kernel is comparatively, that would probably make our mutexes extremely "unfair." This would not violate the POSIX specification, but it does seem like a dangerous choice to make in implementing these APIs. Linux's "futex" API, which our API bears some similarities to, requires at least one atomic test-and-set for an uncontended acquisition, and multiple atomics more for even the simplest case of contended acquisition. If it works for them, it should work for us, too. Fixes #18436. Change-Id: Ib8c28acf04ce03234fe738e41aa0969ca1917540 Reviewed-on: https://review.haiku-os.org/c/haiku/+/6537 Tested-by: Commit checker robot <[email protected]> Reviewed-by: Adrien Destugues <[email protected]> Reviewed-by: waddlesplash <[email protected]>
Haiku
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Haiku is an open-source operating system that specifically targets personal computing. Inspired by the BeOS, Haiku is fast, simple to use, easy to learn and yet very powerful.
Goals
- Sensible defaults with minimal configuration required.
- Clean, clear, concise code.
- Unified desktop environment.
Trying Haiku
Haiku provides pre-built nightly images and release images. Haiku is compatible with a large variety of hardware, but in case you don't want to "take the plunge" and install Haiku on bare metal, you can install it on a virtual machine (VM) instead. If you've never used a VM before, you can follow one of the "Emulating Haiku" guides.
Compiling Haiku
See ReadMe.Compiling.
Contributing
Haiku is a meritocratic open source project with a large variety of tasks. Even if you can't write code, you can still help! Haiku needs designers, (technical) writers, translators, testers... Get involved and help out!
Contributing code
If you're submitting a patch to us, please make sure you're following the patch submitting guidelines.
If you're having trouble finding something in the source tree, you can use one of our web-based source code browsers:
- https://xref.landonf.org/ (OpenGrok, provided by Landon Fuller)
- https://git.haiku-os.org/ (git, provided by Haiku, Inc.)
Contributing documentation
The main piece of documentation that still needs work are the API docs (found
in the tree at docs/user). Just find an undocumented class, write
documentation for it, and submit a patch.
Contributing translations
See wiki:i18n.
Contributing software ports
See HaikuPorts.
Contributing to our infrastructure
See Infrastructure.